spacecraft, but others will require data from multiple instruments on multiple spacecraft. Investigations are further subdivided into measurements.

• Measurements constitute actions that can be undertaken by a specific instrument on a specific spacecraft.

The goals and objectives contained in MEPAG’s 2006 listing of priorities are as follows:

• Goal I. Determine if life ever arose on Mars.

Objective A. Assess the past and present habitability of Mars.

Objective B. Characterize carbon cycling in its geochemical context.

Objective C. Assess whether life is or was present on Mars.

• Goal II. Understand the processes and history of climate on Mars.

Objective A. Characterize Mars’s atmosphere, present climate, and climate processes.

Objective B. Characterize Mars’s ancient climate and climate processes through study of the geologic record.

Objective C. Characterize the state and processes of the martian atmosphere of critical importance for the safe operation of spacecraft.

• Goal III. Determine the evolution of the surface and interior of Mars.

Objective A. Determine the nature and evolution of the geologic processes that have created and modified the martian crust and surface.

Objective B. Characterize the structure, composition, dynamics, and evolution of Mars’s interior.

• Goal IV. Prepare for human exploration.

Objective A. Obtain knowledge of Mars sufficient to design and implement a human mission with acceptable cost, risk, and performance.

Objective B. Conduct risk and/or cost reduction technology and infrastructure demonstrations in transit to, at, or on the surface of Mars.